1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor comprising a conductive substrate on which a photosensitive layer containing a pigment as a photoconductive material, a binder resin, an organic charge-transport agent, and the like is formed.
2. Description of the Prior Art
Conventionally, it has been well known that inorganic materials such as cadmium sulfide and zinc oxide and organic materials such as polyvinyl carbazole are provided as photoconductive materials to be included in photosensitive layers of electrophotographic photoreceptors. In addition, single-layer type photoreceptors have been also known. In this case, a photosensitive layer of each of them is formed as a single film including at least one of the above photosensitive materials.
In recent years, in addition, functionally distinguishable type photoreceptors have been developed as the high sensitive photoreceptors and they have been provided in practical use. In the functionally distinguishable type photoreceptor, the photoconductive functions thereof are divided into two different layers. That is, a charge generation layer for generating charge-carriers by the absorption of exposure and a charge transport layer for holding the electrical potential in dark and transporting the charge carriers.
Furthermore, the photoreceptors using organic materials as main components in their photosensitive layers, especially for their charge transport layers provided as the greater parts thereof, have been studied by many researchers and partially put into practical use by virtue of their advantage features of flexibility and excellent charging abilities, as the characteristics of the organic materials. In addition, they show excellent membrane-formabilities and cost effectiveness in their manufacturing processes.
For obtaining a reproduced image or the like, by the way, an electrophotographic device repeats the cycle of the steps of charging, image exposure, development, transferring, cleaning, and the like to the photoreceptor provided as a constituent part of the device. During the period of repeating the above cycle, the characteristics of the photoreceptor should be kept in stable. In the case of the above photoreceptor using the organic material as one of its main components, especially the functionally distinguishable type photoreceptor comprising a charge transport layer using an organic charge generation material has the excellent properties of charging, photo-sensitivities, and so on. However, these advantages are not enough to satisfy all of the requirements for the photoreceptor. That is, there are much more demands for the photoreceptor that has a high sensitivity in repeat use and long span of life because a decline in the surface potential of the photoreceptor can be observed after repeating the above cycle several times. Thus these problems lead to a decline in image concentration with respect to the image qualities, resulting in being no fit for practical use.
It has not been known what was caused the deterioration but several factors may be related. For example it may be due to ozone, No.sub.x, and other gases to be generated by means of corona discharge. The photoreceptor is usually exposed to the atmosphere caused by the corona discharge during the steps of charging the surface of the photoreceptor, transforming the developed toner on the photoreceptor to paper, and so on. Therefore, the photoreceptor may be gradually deteriorated by repeating the process of image formation, as a result of being affected by the above gases.
In general, the functionally distinguishable type photoreceptor is constructed so as to form a charge transport layer on a charge generation layer for the purpose of stabilizing its electrophotographic characteristics and making its life span longer. For this kind of the construction, the photoreceptor can be used in negative-charged condition where holes are transported as charge carriers by means of the organic charge transport material that has been currently used in practice. Therefore, a negative corona discharge is performed in the step of charging. However, the negative corona discharge leads to the generation of active gases such as ozone and No.sub.x, much more than that in the positive corona discharge, and results in more serious problem of the above deterioration.
Thus the electrophotographic device is adversely affected by the above gases, so that the device generally comprises a means for exhausting gases around the corona discharger. In spite of using such means, however, it is very difficult to remove almost all the gases from the device.
For preventing the deterioration of the photoreceptor, therefore, various anti-oxidizing agents to be added in a photosensitive layer thereof have been proposed. For example hindered phenols are disclosed in Japanese Patent Application Laid-open No. 122444/1982 discloses the addition of hindered phenols, Japanese Patent Application Laid-open No. 143763/1986 discloses the method of adding hindered phenols in large quantities, and Japanese Patent Application Laid-open 105151/19887 discloses the hindered phenol having a specified structure. Besides the hindered phenols, Japanese Patent Application Publication No. 27693/1994 discloses hindered amines and Japanese Patent Application Publication discloses hindered amines and specified electron-acceptable compounds.
As described above, the anti-oxidizing agent prevents the deterioration of the photoreceptor to be caused by the active gases such as ozone and NO.sub.x to a certain extent. In recent years, however, there is a demand for more perfectly preventing the deterioration of the photoreceptor to serve the needs of the photoreceptor having its longer life span in the industrial field.